Single tree record from the 'TREE' {@link DirCache} extension.
A valid cache tree record contains the object id of a tree object and the total number of {@link DirCacheEntry} instances (counted recursively) fromthe DirCache contained within the tree. This information facilitates faster traversal of the index and quicker generation of tree objects prior to creating a new commit.
An invalid cache tree record indicates a known subtree whose file entries have changed in ways that cause the tree to no longer have a known object id. Invalid cache tree records must be revalidated prior to use.